The proposed studies contribute to the resolution of a major unsolved problem in neurobiology: how do precise connections develop between neurons and their targets? Experimental and descriptive studies of a simple vertebrate system, the development of muscle innervation in the chick embryo, have shown that selective pathfinding by the growing tips of individual neurites (the "growth cones") plays an essential role in the genesis of the precise and orderly connections that are required for the nervous system to function. As yet, little is know about the cellular and molecular mechanisms of growth cone guidance. The central aim of the proposed studies is to define the actual cellular mechanisms that mediate growth cone response to navigational cues. This laboratory has recently identified three sources of navigational cues. The mode of action of these cues will be elucidated by studies in culture and in the embryo. 1) The cellular mechanisms that impose a segmental pattern on axonal outgrowth will be defined by time lapse analysis of the interactions of identified motoneurons with cells that have been shown to be the source of the navigational cues for segmentation. 2) Chemotactic cues will be characterized using a novel in vivo assay that will clarify the role of population-specific cues to the development of precise innervation patterns. 3) Hypothesis concerning cellular interactions that many mediate mesenchymal cell death, muscle formation and specific pathway selection in the limb will be tested with ultrastructural analysis and embryonic surgeries. Analysis of the cellular mechanisms that are effective in specific neuronal pathfinding in the chick should give insights into normal and abnormal development of the human nervous system. Factors important to the development of the complexly interdependent neuromuscular system may eventually be manipulated to treat deficiencies in nerve-muscle interactions that are responsible for human disease states. Finally, an understanding of the processes conferring specificity on developing embryonic connections is relevant to an understanding and proper treatment of the relatively poor specific nerve regeneration in humans.